SC2447 View Datasheet(PDF) - Semtech Corporation
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SC2447 Datasheet PDF : 26 Pages
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SC2447
POWER MANAGEMENT
Application Information (Cont.)
Only the voltages in boldface can be precisely set with
standard 1% resistors.
From this table, one may also observe that when the
value
1− h = Vo − 0.5 .
h
0.5
and its multiples fall into the standard resistor value
chart (1%, 5% or so), it is possible to use standard value
resistors to set up the exact and required output voltage
value.
The slope of the compensation ramp is then
Se=0.4*fs.
The slope of the internal compensation ramp is well above
the minimal slope requirement for current loop stability
and is sufficient for all the applications.
With the inner current loop stable, the output voltage is
then regulated with the outer voltage feedback loop. A
simplified equivalent circuit model of the synchronous
Buck converter with current mode control is shown in
Figure 15.
The input bias current of the error amplifier also causes
an error in setting the output voltage. The maximum
inverting input bias current of the error amplifiers is -
380nA. Assuming the non-inverting input is tied to the
0.5V reference output, the percentage error in the
second output voltage will be –100% · (0.38µA) ·
Ro1Ro2 /[0.5 · (Ro1+Ro2) ]. To keep this error below
0.2%, Ro2 < 2.6kΩ.
Loop Compensation
k
SC2447 uses current-mode control for both step-down
channels. Current-mode control is a dual-loop control
system in which the inductor peak current is loosely
controlled by the inner current-loop. The higher gain outer
loop regulates the output voltage. Since the current loop
makes the inductor appear as a current source, the complex
high-Q poles of the output LC network are split into a
dominant pole determined by the output capacitor and
the load resistance and a high frequency pole. This pole-
splitting property of current-mode control greatly simplifies
loop compensation.
The inner current-loop is unstable (sub-harmonic
oscillation) unless the inductor current up-slope is steeper
than the inductor current down-slope. For stable
operation above 50% duty-cycle, a compensation ramp
is added to the sensed-current. In the SC2447 the
compensation ramp is approximately
Figure 15. A Simple Model of Synchronous Buck
Converter with Current Mode Control
The transconductance error amplifier (in the SC2447)
has a gain g of 170µA/V. The target of the compensation
m
design is to select the compensation network consisting
of C2, C3 and R2, along with the feedback resistors Ro1,
Ro2 and the current sensing gain, such that the converter
output voltage is regulated with satisfactory dynamic
performance.
Vramp=D*0.4V.
© 2006 Semtech Corp.
22
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